The art is replete with methods for presulfiding hydrofining catalysts which contain cobalt oxide and/or nickel oxide plus molybdenum oxide and/or tungsten oxide. The overall objective is to temper the "wild" initial activity of the oxide-form catalyst, thereby reducing the deactivation rate of the catalyst, and usually improving its activity for desulfurization and denitrogenation.
Perhaps the most widely used procedure involves contacting the catalyst with a gaseous mixture of hydrogen and H.sup.2 S at elevated temperatures. The presence of hydrogen appears to give a more active catalyst, apparently by maintaining the Group VIB metal sulfide component in an optimum valence state. However, the use of hydrogen in such processes presents certain problems. At elevated temperatures, above about 500.degree. F. (which are normally required to complete the sulfiding) hydrogen in the absence of H.sub.2 S tends to reduce some of the active metal oxides to free metals, resulting in agglomeration, particularly with respect to molybdenum. When a mixed gas stream of H.sub.2 -H.sub.2 S is passed through a deep bed of catalyst, all of the H.sub.2 S is initially chemisorbed or combined with the upper layers of the catalyst bed, leaving the lower portion of the bed substantially free of H.sub.2 S. It is therefore necessary to first sulfide the entire bed of catalyst at relatively low temperatures, and then gradually raise temperatures to complete the sulfiding. Another difficulty with gas phase presulfiding is that the reaction is exothermic, and depending on metals concentration, can generate very high instantaneous temperatures at the active sites, resulting in a lowering of activity.
It is known in the art that the foregoing difficulties can be substantially alleviated by presulfiding with hydrogen and a hydrocarbon feedstock containing native organic sulfur compounds and/or added organic sulfur compounds such as mercaptans, thioethers, carbon disulfide, thiophene and the like. By contacting such sulfur-containing feedstocks with the catalyst under mile conditions, such that the conversion of organic sulfur compounds to H.sub.2 S is incomplete, the generation of H.sub.2 S will take place throughout all parts of the catalyst bed, thereby preventing reduction of the active metal oxides. Also, the presence of unreacting hydrocarbons provides a heat sink, thereby preventing local overheating.
The present invention represents an improvement over all the foregoing methods. We have discovered that a catalyst of improved stability and activity is produced when the catalyst is sulfided using mixed sulfiding agents comprising (1) a heavy liquid phase mineral oil fraction containing at least about 0.5 wt.% of native organic sulfur, and (2) a gaseous H.sub.2 -H.sub.2 S mixture containing about 0.3%-10% by volume of H.sub.2 S. Catalysts sulfided with these mixtures are found to display an initially quite rapid deactivation rate which, most unexpectedly, levels out to a very low rate after a few days on stream. While we are unable to account with any degree of certainty for this surprising result, it is possible that it may be related to the temperature profile prevailing in the catalyst bed during presulfiding.
In an adiabatic reactor containing a substantial bed depth of catalyst, the temperature profiles generated during presulfiding can be fairly complex. Hydrogen sulfide itself generates an exotherm which travels slowly down the catalyst bed as sulfiding progresses. Sulfiding with H.sub.2 S generated by the decomposition of organic sulfur compounds native to mineral oil feedstocks tends to generate a gradually ascending temperature profile downwardly through the reactor. Sulfiding via the decomposition of easily decomposable added organic sulfur compounds such as mercaptans generates two types of exotherms: the first remains relatively stationary near the top of the catalyst bed and is attributable to the hydrocracking of the sulfur compound to form H.sub.2 S; the second is attributable to the generated H.sub.2 S combining with the catalytic metals, and moves gradually downward in the reactor as the catalyst becomes sulfided. The process of this invention provides a combination of a gradually ascending temperature profile due to desulfurization of feedstock, and a downwardly travelling exotherm due to the added H.sub.2 S, with no stationary exotherm. It is conceivable that this represents an optimum temperature profile for presulfiding.
It should be noted that in some prior art processes for presulfiding with hydrocarbon feedstocks, the effluent gas phase is continuously recycled. If this gas phase contained H.sub.2 S, it would appear that the benefits of the present invention would automatically be obtained. However, some prior art disclosures (e.g. U.S. Pat. No. 3,948,763) suggest removing such H.sub.2 S from the recycle gas, and in any event very little H.sub.2 S, i.e., less than about 20 ppm, would normally be present in such gases while the catalyst is still being actively sulfided. Maximum benefits of the present invention are not obtained unless H.sub.2 S is present in the influent gases substantially before the catalyst is completely sulfided, as evidenced by the presence of substantially the same amount of total sulfur in the effluent from the catalyst bed as is being fed thereto under sulfiding conditions. We do not exclude however the obtaining of some substantial benefits of the invention by initiating the presulfiding with feedstock alone (and hydrogen) and adding H.sub.2 S to the influent gases at some later time, but before completion of the sulfiding.
Data submitted hereinafter will show that the claimed presulfiding method is superior to:
(1) presulfiding with gaseous H.sub.2 -H.sub.2 S mixtures;
(2) presulfiding with feedstock alone; and
(3) presulfiding with the feedstock plus an added mercaptan.